Copied from a post I wrote on http://groups.yahoo.com/group/solaroof/ on the 25th November 2005
a guy who visited my site sent me
i'm now thinking of using such a system to circulate the plane water from my existing thermal mass. i would then place a snake of 4" tube inside the tank through which i would circulate air from the building environment. another option would be to pump the thermal mass water in a hose on the ground. thus warming the plant roots where they need it. of course i may just put all the thermal gain into making warm bubbles, but i'm still unsure if this is the most effect way of using all the thermal energy in a single cavity building.
its been atypically cold here for november and i have been able to make some initial observations on my project. until i have my full monitoring installed however, i am resisting coming to any solid conclusions. like others have already noted here, i'm now unconvinced warming the bubbles is the most effective means of transferring thermal energy into the building. at least if it is a greenhouse and the plants are requiring warm roots.
i'm sure now i want to partition my build and cavities to enable me to make direct comparisons using different systems. i'm convinced this is the only way to created controlled tests. i will partition the interior with thick solid insulation board and partition the cavities with two poly tubes pumped full of polystyrene beads. these tubes will be sealed of course. i'll then need to install another identical bubble generator to make a symmetric system.
one problem i am having with my system is the inadequate transfer of thermal energy from my soap tank to my thermal mass during collection mode. as my soap tank is created from a bladder with a rolled stabled seam, this seam cannot go below the surface of my thermal mass water. thus it only transfers its thermal energy to the top 15% of the thermal mass. on a sunny day i can bring the top of my thermal mass up to 18C, but only a foot beneath the surface the temp is only 11C. at the bottom of the thermal mass at a depth of 1 metre is cooler still. i'm guessing the water is stratifying being as it is being warmed from the top. if the soap tank was at the bottom of the thermal mass, i don't think this would be the case. this is why we have elements in the bottoms of tanks right? cold water is more dense than warm.
so once my soap tank gets up to 18C and the top of my thermal mass is 18C it stops giving up its heat and the efficiency of the collection system is reduced. if the soap film is going to the cavity at 18C it will come back at 18C. if it goes to the cavity at 15C it will come back at 18C. so if we can transfer the thermal energy quickly to the cooler larger thermal mass then we can run the collection more efficiently and collect more energy. this is understanding that 1000ltrs at 16C contains more thermal energy than 150ltrs at 18C.
i'm now starting to think that copper pipe might be the best solution as suggested by ross. my bladder is made from a heavy weight polythene, and even the transfer of thermal energy to the top of the thermal mass when i first turn the collection system on is slower than i expected. i think it is a surface area issue. although i also think the transfer of thermal energy through a poly skin is slower than one might imagine. this is another issue that makes me wonder if there is not more efficient ways of warming our interior space rather than via a poly skin.
another conclusion i am coming to is that any part of a solaroof building that holds standing liquid, be it a gutter or tank, needs to hard bodied. my soap return troughs at the base of my cavities are soft poly surrounded in foam insulation and steel, so you would think they were pretty safe from damage. not so. twice now i have dropped a screw driver into my cavity space where it has slid down to the bottom puncturing the trough. the problem with soft guttering is that we need to dry it and then make a patch which will last years. try doing this in driving rain and cold. also we need to connect plumbing to our soft gutter. i've finally made what is a very water tight job, but after a few years, i'm not sure how it will fair.
saying this, i'm sure solaroof works on all levels to some degree, and this is why i am investing time and money into trying to understand and realise its full potential, at least within the limitations of my back yard. this can only be done with a functioning building and rigorous monitoring and testing. having operated my build for the first time in cold conditions i'm now sure i would not undertake to design any more buildings until the data at the end of the winter has been looked at in depth and accurate efficiency models can be predicted. if i don't have this capacity, then i feel i could waste a lot of time, resources and effort.